Ballcocks!

Was the "main stopcock" fully open before you turned it off? It may help if you check the flow a little.

Or you may have an airlock, which will probably clear itself in due course.

I always count how many turns are required to turn the mains off. Sometimes people have had to tune them to keep things quiet.

Thanks. I've tried lots of setting of the stopcock. Even when nearly closed it still whistles like a good'un. Not sure where than can be an airlock. The mains water feed only goes to the loft water tank and the kitchen cold tap.

Some ballcocks (float valve) have an adjusting screw (for water level) You could try adjusting that. You could try fitting a dip tube (stops the wave created by splashing water) Or change the washer in the ballcock. There may be a bit of crap got in the ballcock jet/orifice. (Disturbed when you turned off the water. When you get the slug out (that holds the washer), clean it up with wire wool. Check the play in the slot where the float valve arm fits into. It may be excessive. In the end you may need a new ball/float valve.

BTW. Note you lovers of ball valves out there. His service valve didn't work. Surprise surprise.

Is your new service valve the same as the old ie a full flow type or reduced diameter type? If not the change in bore through the valve could be causing the problem.

You could try something like a torbeck valve, they stay full on until they close. They have a pressure powered "motor" inside.

Thanks. The new service valve is for the new (yet to be delivered) dishwasher. I don't think it's relevant with regards to the whistling ballcock.

I could. Are there any makes that you could recommend / avoid? Ta.

I still like the idea of an electrically controlled valve. I have seen circuits on the web which actually match one I created myself.